For improving the driving stability of a pneumatic tire on a wet road surface, enhancement of the edge effect is effective, and the ensuring of the block rigidity is also important at the same time. Meanwhile, one of the significant factors for ensuring the snow traction is increase in a volume of grooves. For instance, in a case where the volume of grooves in the tread portion of the pneumatic tire is increased, the snow traction can be improved, whereas the block rigidity is decreased, so that the driving stability on a wet road surface is deteriorated. As described above, the conditions for improving the driving stability on a wet road surface and the snow performance are in a trade-off relationship.
Against this background, numerous proposals have been heretofore made on tread patterns each exhibiting a good snow performance (see Patent Document 1 and Patent Document 2, for instance). However, it is still difficult to achieve both the driving stability on a wet road surface and the snow performance by using these tread patterns.    Patent Document 1: Japanese patent application Kokai publication No. Hei 9-216504    Patent Document 2: Japanese patent application Kokai publication No. 2003-146020